It is known that vehicle wheels are generally constituted by a metallic rim which is provided, on its peripheral region, with annular flanges between which an elastic tire is keyed; the end portions of the tire, known as beads, each abut against a respective flange of the rim.
To perform the operations for fitting and removing tires and the corresponding rims, so-called tire changing machines are currently used which allow to remove the tire from the corresponding rim in order to perform for example maintenance or replacement of the inner tube and to subsequently refit the same tire or a replacement tire on the wheel rim.
Particularly strong and sturdy automatic tire changing machines are known for fitting/removing very heavy and bulky wheels, such as wheels for trucks, farming tractors, earth-moving machines, or such as special wheels with diameters of three meters or more.
These machines are constituted substantially by a supporting frame for means for coupling and turning, about a horizontal axis, a rim on/from which the tire is to be fitted/removed and for a working assembly provided with a working head for fitting and removing the tire.
Such working head is generally provided with a bead breaker tool, which is suitable to be inserted between the beads of the tire and the corresponding annular flanges of the rim in order to separate them, and an extractor tool, which is provided with a curved end part suitable to engage the beads of the tire and move them away from the corresponding rim.
In a first type of tire changing machine for very heavy and bulky wheels, the coupling and turning means are supported on an elongated rigid structure, which is associated with the frame so that it can rotate about an oscillation axis that is parallel to the rim rotation axis; accordingly, such means can be moved along a circular path that lies along a plane that is perpendicular to the rotation axis.
The working head is instead mounted on a working arm, which has a preset length and can slide with respect to the frame by means of a slider engaged along a straight guide, which is rigidly associated with the frame and is parallel to the rim rotation axis.
During fitting and removal operations, the direction determined by the working arm is substantially perpendicular to the rotation axis and its inclination with respect to the ground, which is fixed and determined during production, is generally approximately 39°.
In these tire changing machines, the supporting structure of the coupling and rotation means can be turned about the oscillation axis in order to change its distance from the ground and facilitate the coupling/separation of the rim.
Further, the movement of the elongated structure is important in order to move the rim rotation axis toward/away from the working head and arrange the wheel in the correct working position, i.e., the position in which the circumferential portion of the rim is arranged proximate to the tools.
These known types of machine are susceptible of further improvements aimed in particular at increasing their flexibility and efficiency in use.
Such machines in fact have working arms which are inconveniently long, their dimensions being determined by the need to arrange the working head at such a distance from the ground as to allow to work even with the largest special wheels, the rims of which must be arranged at a considerable height due to the considerable radial dimensions of the tires.
In practice, this forces the user to arrange even the smaller wheels at considerable heights.
It should be noted in fact that truck wheels or wheels for farming tractors, which have substantial weights and dimensions but not as much as special wheels having a diameter of three meters, are arranged so that their tread lies at a distance from the ground that sometimes exceeds 60 centimeters.
Fitting/removing tires in such conditions, in addition to being particularly awkward and scarcely practical, is also considerably dangerous for the safety of the operator, who might be struck by the falling wheel if the rim disengages accidentally from the coupling and rotation means.